The role of phosphatidylcholine in fatty acid exchange and desaturation in Brassica napus L. leaves

Abstract

The role of phosphatidylcholine (PC) in fatty acid exchange and desaturation was examined and compared with that of monogalactosyldiacylglycerol (MGDG) in Brassica napus leaves using (14)C-labelling in vivo. Data are presented which indicate that in the chloroplast newly formed saturated (palmitic acid, 16:0) and monounsaturated (oleic acid, 18:1) fatty acid is incorporated into MGDG and desaturated in situ. In the non-plastidic compartments, however, newly formed fatty acid is exchanged with polyunsaturated fatty acid in PC, the probable major site of subsequent desaturation. The unsaturated fatty acid is released to the acyl-CoA pool, which is then used to synthesize diacylglycerol (DAG) containing a high level of unsaturated fatty acid. This highly unsaturated DAG may be the source for the biosynthesis of other cellular glycerolipids. The generally accepted pathway in which PC is synthesized from molecular species of DAG containing 16:0 and 18:1 followed by desaturation of the 18:1 to linoleic (18:2) and linolenic (18:3) acids is questioned.

Fatty acid neutral losses observed in tandem mass spectrometry with collision‐induced dissociation allows regiochemical assignment of sulfoquinovosyl‐diacylglycerols

“…DAGs originated within the chloroplasts usually bear a 18:1 acyl chain on the sn ‐1 position and a 16:0 one on the sn ‐2 position, a combination corresponding to a typical prokaryotic‐type cell . On the contrary, DAGs generated through a eukaryotic pathway usually contain a mixture of 18:2 and 18:3 side chains at both sn ‐1 and sn ‐2 positions and a 16:0 chain in the sn ‐1 position . The cited moieties were those actually found in the ten most abundant SQDGs identified in the present work.…”

“…Half of the 24 identified species bear a 16:0 acyl chain on the sn ‐2 position, and five of them are included among the ten most abundant components . The nature and position of acyl chains in the SQDGs can provide useful information on their biosynthesis in higher plants, which is known to occur within plastids . Actually, SQDG synthase can use either DAGs formed via the chloroplastic (prokaryotic) pathway or those imported from the endoplasmic reticulum (eukaryotic pathway) .…”

“…Actually, SQDG synthase can use either DAGs formed via the chloroplastic (prokaryotic) pathway or those imported from the endoplasmic reticulum (eukaryotic pathway) . DAGs originated within the chloroplasts usually bear a 18:1 acyl chain on the sn ‐1 position and a 16:0 one on the sn ‐2 position, a combination corresponding to a typical prokaryotic‐type cell . On the contrary, DAGs generated through a eukaryotic pathway usually contain a mixture of 18:2 and 18:3 side chains at both sn ‐1 and sn ‐2 positions and a 16:0 chain in the sn ‐1 position .…”

The plastid outer membrane localized LPTD1 is important for glycerolipid remodelling under phosphate starvation

“…Chloroplasts lipids are synthesized by fatty acids from the ER or plastids, respectively (Browse et al ). PC (Tanaka et al ), LPC (Jessen et al ), DAG (Williams et al ) and PA (Wang et al ) are discussed as lipids that are imported from ER to plastids. Consistent with the observed recognition of PA by atTGD4 in vitro (Wang et al ), PA synthesized in the ER is likely transported into plastids involving atTGD4 (Fig.…”

Phosphatidylcholine is transferred from chemically‐defined liposomes to chloroplasts through proteins of the chloroplast outer envelope membrane

“…In vivo pulse chase studies indicate that PC (detected in envelope/chloroplast) synthesized in the ER could be the immediate precursor for galactolipid synthesis in chloroplasts [15,16,17,30,31]. In addition, diacylglycerol (DAG) [36], phosphatic acid (PA) [41] and lyso‐PC [23] have been suggested to be galactolipid precursors transported from the ER. Since chloroplasts lack capacity to synthesize the head group of PC, and both the outer envelope and the ER contain a significant amount of PC it seems likely that PC is transported from the ER whether or not it is the major precursor for galactolipids.…”